HIGHWAY DESIGN MANUAL 400-1 March 7, 2014...402.1 Capacity Adequate capacity to handle peak period...

HIGHWAY DESIGN MANUAL 400-1 March 7, 2014

CHAPTER 400 INTERSECTIONS AT GRADE

Intersections are planned points of conflict where two

or more roadways join or cross. At-grade

intersections are among the most complicated

elements on the highway system, and control the

efficiency, capacity, and safety for motorized and

non-motorized users of the facility. The type and

operation of an intersection is important to the

adjacent property owners, motorists, bicyclists,

pedestrians, transit operators, the trucking industry,

and the local community.

There are two basic types of at grade intersections:

crossing and circular. It is not recommended that

intersections have more than four legs. Occasionally,

local development and land uses create the need for a

more complex intersection design. Such intersections

may require a specialized intersection design to

handle the specify traffic demands at that location. In

addition to the guidance in this manual, see Traffic

Operations Policy Directive (TOPD) Number 13-02:

Intersection Control Evaluation (ICE) for direction

and procedures on the evaluation, comparison and

selection of the intersection types and control

strategies identified in Index 401.5. Also refer to the

Complete Streets Intersection Guide for further

information.

Topic 401 - Factors Affecting Design

Index 401.1 - General

At-grade intersections must handle a variety of

conflicts among users, which includes truck, transit,

pedestrians, and bicycles. These recurring conflicts

play a major role in the preparation of design stan-

dards and guidelines. Arriving, departing, merging,

turning, and crossing paths of moving pedestrians,

bicycles, truck, and vehicular traffic have to be

accommodated within a relatively small area. The

objective of designing an intersection is to effectively

balance the convenience, ease, and comfort of the

users, as well as the human factors, with moving

traffic (automobiles, trucks, motorcycles, transit

vehicles, bicycles, pedestrians, etc.).The safety and

mobility needs of motorist, bicyclist and pedestrians

as well as their movement patterns in intersections

must be analyzed early in the planning phase and then

followed through appropriately during the design

phase of all intersections on the State highway. It is

Departmental policy to develop integrated

multimodal projects in balance with community

goals, plans, and values.

The Complete Intersections: A Guide to

Reconstructing Intersections and Interchanges for

Bicyclists and Pedestrians contains a primer on the

factors to consider when designing intersections. It is

published by the California Division of Traffic

Operations.

401.2 Human Factors

(1) The Driver. An appreciation of driver

performance is essential to proper highway

design and operation. The suitability of a design

rests as much on how safely and efficiently

drivers are able to use the highway as on any

other criterion.

Motorist’s perception and reaction time set the

standards for sight distance and length of

transitions. The driver’s ability to understand

and interpret the movements and crossing times

of the other vehicle drivers, bicyclists, and

pedestrians using the intersection is equally

important when making decisions and their

associated reactions. The designer needs to keep

in mind the user’s limitations and therefore

design intersections so that they meet user

expectation.

(2) The Bicyclist. Bicyclist experience, skills and

physical capabilities are factors in intersection

design. Intersections are to be designed to help

bicyclists understand how to traverse the

intersection. Chapter 1000 provides intersection

guidance for Class I and Class III bikeways that

intersect the State highway system. The

guidance in this chapter specifically relates to

bicyclists that operate within intersections on

the State highway system.

(3) The Pedestrian. Understanding how pedestrians

will use an intersection is critical because

pedestrian volumes, their age ranges, physical

ability, etc. all factor in to their startup time and

the time it takes them to cross an intersection

and thus, dictates how to design the intersection

to avoid potential conflicts with bicyclists and

motor vehicles. The guidance in this chapter

specifically relates to pedestrian travel within

400-2 HIGHWAY DESIGN MANUAL

March 7, 2014

intersections on the State highway system. See

Topic 105, Pedestrian Facilities, Design

Information Bulletin 82 - “Pedestrian

Accessibility Guidelines for Highway Projects,”

the AASHTO Guide for the Planning, Design,

and Operation of Pedestrian Facilities, and the

California Manual on Uniform Traffic Control

Devices (California MUTCD) for additional

guidance.

401.3 Traffic Considerations

Good intersection design clearly indicates to

bicyclists and motorists how to traverse the

intersection (see Figure 403.6A). Designs that

encourage merging traffic to yield to through bicycle

and motor vehicle traffic are desirable.

The size, maneuverability, and other characteristics

of bicycles and motorized vehicles (automobiles,

trucks, transit vehicles, farm equipment, etc.) are all

factors that influence the design of an intersection.

The differences in operating characteristics between

bicycles and motor vehicles should be considered

early in design.

Table 401.3 compares vehicle characteristics to

intersection design elements.

A design vehicle is a convenient means of

representing a particular segment of the vehicle

population. See Topic 404 for a further discussion of

the uses of design vehicles.

Transit vehicles and how their stops interrelate with

an intersection, pedestrian desired walking patterns

and potential transfers to other transit facilities are

another critical factor to understand when designing

an intersection. Transit stops and their placement

needs to take into account the required maintenance

operations that will be needed and usually supplied

by the Transit Operator.

401.4 The Physical Environment

In highly developed urban areas, where right of way

is usually limited, the volume of vehicular traffic,

pedestrians, and bicyclists may be large, street

parking exists, and transit stops (for both buses and

light rail) are available. All interact in a variety of

movements that contribute to and add to the

complexity of a State highway and can result in busy

intersections.

Industrial development may require special attention

to the movement of large trucks.

Rural areas where farming occurs may require

special attention for specialized farm equipment. In

addition, rural cities or town centers (rural main

streets) also require special attention.

Rural intersections in farm areas with low traffic

volumes may have special visibility problems or

require shadowing of left-turn vehicles from high

speed approach traffic.

Table 401.3

Vehicle Characteristics

Intersection Design

Element Affected

Length

Length of storage lane

Width

Lane width

Height Clearance to overhead

signs and signals

Wheel base Corner radius and width

of turning lanes

Acceleration Tapers and length of

acceleration lane

Deceleration Tapers and length of

deceleration lane

There are many factors to be considered in the design

of intersections, with the goal to achieve a functional,

safe and efficient intersection for all users of the

facility. The location and level of use by various

modes will have an impact on intersection design,

and therefore should be considered early in the design

process. In addition to current levels of use, it is

important to consider future travel patterns for

vehicles, including trucks; pedestrian and bicycle

demand and the future expansion of transit.

401.5 Intersection Type

Intersection types are characterized by their basic

geometric configuration, and the form of intersection

traffic control that is employed:


(1) Geometric Configurations

(a) Crossing-Type Intersections - “Tee” and 4-

legged intersections

(b) Circular Intersections –roundabouts, traffic

circles, rotaries; however, only roundabouts

are acceptable for State highways.

(c) Alternative Intersection Designs – various

effective geometric alternatives to traditional

designs that can reduce crashes and their

severity, improve operations, reduce

congestion and delay typically by reducing

or altering the number of conflict points;

these alternatives include geometric design

features such as intersections with displaced

left-turns or variations on U-turns.

(2) Intersection Control strategies, See California

MUTCD and Traffic Operations Policy Directive

(TOPD) Number 13-02, Intersection Control

Evaluation for procedures and guidance on how

to evaluate, compare and select from among the

following intersection control strategies:

(a) Two-Way Stop Controlled - for minor road

traffic

(b) All-Way Stop Control

(c) Signal Control

(d) Yield Control (Roundabout)

Historically, crossing-type intersections with signal

or “STOP”-control have been used on the State

highway system. However, other intersection types,

given the appropriate circumstances may enhance

intersection performance through fewer or less

severe crashes and improve operations by reducing

overall delay. Alternative intersection geometric

designs should be considered and evaluated early in

the project scoping, planning and decision-making

stages, as they may be more efficient, economical and

safer solutions than traditional designs. Alternative

intersection designs can effectively balance the

safety and mobility needs of the motor vehicle

drivers, transit riders, bicyclists and pedestrians using

the intersection.

401.6 Transit

Transit use may range from periodic buses, handled

as part of the normal mix of vehicular traffic, to Bus

Rapid Transit (BRT) or light rail facilities which can

have a large impact on other users of the intersection.

Consideration of these modes should be part of the

early planning and design of intersections.

Topic 402 - Operational Features Affecting Design

402.1 Capacity

Adequate capacity to handle peak period traffic

demands is a basic goal of intersection design.

(1) Unsignalized Intersections. The “Highway

Capacity Manual”, provides methodology for

capacity analysis of unsignalized intersections

controlled by “STOP” or “YIELD” signs. The

assumption is made that major street traffic is

not affected by the minor street movement.

Unsignalized intersections generally become

candidates for signalization when traffic

backups begin to develop on the cross street or

when gaps in traffic are insufficient for drivers

to yield to crossing pedestrians. See the

California MUTCD, for signal warrants.

Changes to intersection controls must be

coordinated with District Traffic Branch.

(2) Signalized Intersections. See Topic 406 for

analysis of simple signalized intersections,

including ramps. The analysis of complex and

alternative intersections should be referred to

the District Traffic Branch; also see Traffic

Operations Policy Directive (TOPD) Number

13-02.

(3) Roundabout Intersections. See TOPD Number

13-02 for screening process and the Intersection

Control Evaluation(ICE) Process Informational

Guide for operational analysis methods and

tools.

402.2 Collisions

(1) General. Intersections have a higher potential

for conflict compared to other sections of the

highway because travel is interrupted, traffic

streams cross, and many types of turning

movements occur.

The type of traffic control affects the type of

collisions. Signalized intersections tend to have

more rear end and same-direction


July 1, 2015

sideswipes than intersections with “STOP”-

control on minor legs. Roundabouts experience

few angle or crossing collisions. Roundabouts

reduce the frequency and severity of collisions,

especially when compared to the performance

of signalized intersections in high speed

environments. Other alternative intersection

types are configurations to consider for

minimizing the number of conflict points.

(2) Undesirable Geometric Features.

• Inadequate approach sight distance.

• Inadequate corner sight distance.

• Steep grades.

• Five or more approaches.

• Presence of curves within

intersections(unless at roundabouts).

• Inappropriately large curb radii.

• Long pedestrian crossing distances.

• Intersection Angle <75 degrees (see Topic

403).

402.3 On-Street Parking

On-street parking generally decreases through-traffic

capacity, impedes traffic flow, and increases crash

potential. Where the primary service of the arterial is

the movement of vehicles, it may be desirable to

prohibit on-street parking on State highways in urban

and suburban expressways and rural arterial sections.

However, within urban and suburban areas and in

rural communities located on State highways, on-

street parking should be considered in order to

accommodate existing land uses. Where adequate

off-street parking facilities are not available, the

designer should consider on-street parking, so that

the proposed highway improvement will be

compatible with the land use. On-street parking as

well as off-street parking needs to comply with

DIB82. See AASHTO, A Policy on Geometric

Design of Highways and Streets for additional

guidance related to on-street parking.

402.4 Consider All Users

Intersections should accommodate all users of the

facility, including vehicles, bicyclists, pedestrians

and transit. Bicycles have all the rights and

responsibilities as motorist per the California

Vehicle Code, but should have separate consideration

of their needs, even separate facilities if volumes

warrant. Pedestrians should not be prohibited from

crossing one or more legs of an intersection, unless

no other safe alternative exists. Pedestrians can be

prohibited from crossing one or more legs of an

intersection if a reasonable alternate route exists and

there is a demonstrated need to do so. All pedestrian

facilities shall be ADA compliant as outlined in DIB

82. Transit needs should be determined early in the

planning and design phase as their needs can have a

large impact on the performance of an intersection.

Transit stops in the vicinity of intersections should be

evaluated for their effect on the safety and operation

of the intersection(s) under study. See Topic 108 for

additional information.

402.5 Speed-Change Areas

Speed-change areas for vehicles entering or leaving

main streams of traffic are beneficial to the safety and

efficiency of an intersection. Entering traffic merges

most efficiently with through traffic when the

merging angle is less than 15 degrees and when speed

differentials are at a minimum.

Topic 403 - Principles of Channelization

403.1 Preference to Major Movements

The provision of direct free-flowing high-standard

alignment to give preference to major movements is

good channelization practice. This may require some

degree of control of the minor movements such as

stopping, funneling, or even eliminating them. These

controlling measures should conform to natural paths

of movement and should be introduced gradually to

promote smooth and efficient operation.

403.2 Areas of Conflict

Large multilane undivided intersection areas are

undesirable. The hazards of conflicting movements

are magnified when motorists, bicyclists, and

pedestrians are unable to anticipate movements of

other users within these areas. Channelization

reduces areas of conflict by separating or regulating

traffic movements into definite paths of travel by the

use of pavement markings or traffic islands.

HIGHWAY DESIGN MANUAL 400-5 December 30, 2015

Multilane undivided intersections, even with

signalization, are more difficult for pedestrians to

cross. Providing pedestrian refuge islands enable

pedestrians to cross fewer lanes at a time.

See Index 403.7 for traffic island guidance when used

as pedestrian refuge. Curb extensions shorten

crossing distance and increase visibility. See Index

303.4 for curb extensions.

403.3 Angle of Intersection

A right angle (90°) intersection provides the most

favorable conditions for intersecting and turning

traffic movements. Specifically, a right angle

provides:

• The shortest crossing distance for motor

vehicles, bicycles, and pedestrians.

• Sight lines which optimize corner sight distance

and the ability of motorists to judge the relative

position and speed of approach traffic.

• Intersection geometry that can reduce vehicle

turning speeds so collisions are more easily

avoided and the severity of collisions are

minimized.

• Intersection geometry that sends a message to

turning bicyclists and motorists that they are

making a turning movement and should yield as

appropriate to through traffic on the roadway

they are leaving, to traffic on the receiving

roadway, and to pedestrians crossing the

intersection.

Minor deviations from right angles are generally

acceptable provided that the potentially detrimental

impact on visibility and turning movements for large

trucks (see Topic 404) can be mitigated. However,

large deviations from right angles may decrease

visibility, hamper certain turning operations, and will

increase the size of the intersection and therefore

crossing distances for bicyclists and pedestrians, may

encourage high speed turns, and may reduce yielding

by turning traffic. When a right angle cannot be

provided due to physical constraints, the interior

angle should be designed as close to 90 degrees as is

practical, but should not be less than 75 degrees.

Mitigation should be considered for the affected

intersection design features. (See Figure 403.3A). A

75 degree angle does not unreasonably increase the

crossing distance or generally decrease visibility.

Class II bikeway crossings at railroads follow similar

guidance to Class I bikeway crossings at railroads,

see Index 1003.5(3), and Figure 403.3B.

A characteristic of skewed intersection angles is that

they result in larger intersections.

When existing intersection angles are less than

75 degrees, the following retrofit improvement

strategies should be considered:

• Realign the subordinate intersection legs if the

new alignment and intersection location(s) can

be designed without introducing new geometric

or operational deficiencies.

• Provide acceleration lanes for difficult turning

movements due to radius or limited visibility.

• Restrict problematic turning movements; e.g. for

minor road left turns with potentially limited

visibility.

• Provide refuge areas for pedestrians at very long

crossings.

For additional guidance on the above and other

improvement strategies, consult with the District

Design Liaison.

Particular attention should be given to skewed angles

on curved alignment with regards to sight distance

and visibility. Crossroads skewed to the left have

more restricted visibility for drivers of vans and

trucks than crossroads skewed to the right. In

addition, severely skewed intersection angles,

coupled with steep downgrades (generally over

4 percent) can increase the potential for high centered

vehicles to overturn where the vehicle is on a

downgrade and must make a turn greater than

90 degrees onto a crossroad. These factors should be

considered in the design of skewed intersections.

403.4 Points of Conflict

Channelization separates and clearly defines points

of conflict within the intersection. Bicyclists,

pedestrians and motorists should be exposed to only

one conflict or confronted with one decision at a time.

Speed-change areas for diverging traffic should

provide adequate length clear of the through lanes to

permit vehicles to decelerate after leaving the

through lanes.


March 7, 2014

See AASHTO, A Policy on Geometric Design of

Highways and Streets for additional guidance on

speed-change lanes.

Figure 403.3A Angle of Intersection

(Minor Leg Skewed to the Right)

Figure 403.3.B Class II Bikeway

Crossing Railroad

403.5 (Currently Not In Use)

403.6 Turning Traffic

A separate turning lane removes turning movements

from the intersection area. Abrupt changes in

alignment or sight distance should be avoided,

particularly where traffic turns into a separate turning

lane from a high-standard through facility.

For wide medians, consider the use of offset left-turn

lanes at both signalized and unsignalized

intersections. Opposing left-turn lanes are offset or

shifted as far to the left as practical by reducing the

width of separation immediately before the

intersection. Rather than aligning the left-turn lane

exactly parallel with and adjacent to the through lane,

the offset left-turn lane is separated from the adjacent

through lane. Offset left-turn lanes provide improved

visibility of opposing through traffic. For further

guidance on offset left-turn lanes, see AASHTO, A

Policy on Geometric Design of Highways and

Streets.

(1) Treatment of Intersections with Right-Turn-

Only Lanes. Most motor vehicle/bicycle

collisions occur at intersections. For this

reason, intersection design should be

accomplished in a manner that will minimize

confusion by motorists and bicyclists, eliminate

ambiguity and induce all road users to operate

in accordance with the statutory rules of the road

in the California Vehicle Code. Right-turn-only

lanes should be designed to meet user

expectations and reduce conflicts between

vehicles and bicyclists.

Figure 403.6A illustrates a typical at-grade

intersection of multilane streets without right-

turn-only lanes. Bike lanes or shoulders are

included on all approaches. Some common

movements of motor vehicles and bicycles are

shown. A prevalent crash type is between

straight-through bicyclists and right-turning

motorists, who do not yield to through

bicyclists.

Optional right-turn lanes should not be used in

combination with right-turn-only lanes on roads

where bicycle travel is permitted. The use of

optional right-turn lanes in combination with

right-turn-only lanes is not recommended in any

case where a Class II bike lane is present. This

may increase the need for dual or triple right-

turn-only lanes, which have

HIGHWAY DESIGN MANUAL 400-7 May 7, 2012

Figure 403.6A

Typical Bicycle and Motor Vehicle Movements at Intersections of Multilane Streets without Right-Turn-Only Lanes

NOTE:

Only one direction is shown for clarity.


July 1, 2015

Figure 403.6B

Bicycle Left-Turn-Only Lane

NOTES:

(1) For bicycle lane markings, see the California MUTCD.

(2) Bicycle detectors are necessary for signalized intersections.

(3) Left-turn bicycle lane should have receiving bike lane or shoulder.


challenges with visibility between turning

vehicles and pedestrians. Multiple right-turn-

only lanes should not be free right-turns when

there is a pedestrian crossing. If there is a

pedestrian crossing on the receiving leg of

multiple right-turn-only lanes, the intersection

should be controlled by a pedestrian signal head,

or geometrically designed such that pedestrians

cross only one turning lane at a time.

Locations with right-turn-only lanes should

provide a minimum 4-foot width for bicycle use

between the right-turn and through lane when

bikes are permitted, except where posted speed

is greater than 40 miles per hour, the minimum

width should be 6 feet. Configurations that

create a weaving area without defined lanes

should not be used.

For signing and delineation of bicycle lanes at

intersections, consult District Traffic

Operations.

Figure 403.6B depicts an intersection with a

left-turn-only bicycle lane, which should be

considered when bicycle left-turns are common.

A left-turn-only bicycle lane may be considered

at any intersection and should always be

considered as a tool to provide mobility for

bicyclists. Signing and delineation options for

bicycle left-turn-only lanes are shown in the

California MUTCD.

(2) Design of Intersections at Interchanges. The

design of at-grade intersections at interchanges

should be accomplished in a manner that will

minimize confusion of motorists, bicyclists, and

pedestrians. Higher speed, uncontrolled entries

and exits from freeway ramps should not be

used at the intersection of the ramps with the

local road. The smallest curb return radius

should be used that accommodates the design

vehicle. Intersections with interior angles close

to 90 degrees reduce speeds at conflict points

between motorists, bicyclists, and pedestrians.

The intersection skew guidance in Index 403.3

applies to all ramp termini at the local road.

403.7 Refuge Areas

Traffic islands should be used to provide refuge areas

for bicyclists and pedestrians. See Index 405.4 for

further guidance.

403.8 Prohibited Turns

Traffic islands may be used to direct bicycle and

motorized vehicle traffic streams in desired

directions and prevent undesirable movements. Care

should be taken so that islands used for this purpose

accommodate convenient and safe pedestrian and

bicycle crossings, drainage, and striping options. See

Topic 303.

403.9 Effective Signal Control

At intersections with complex turning movements,

channelization is required for effective signal control.

Channelization permits the sorting of approaching

bicycles and motorized vehicles which may move

through the intersection during separate signal

phases. Pedestrians may also have their own signal

phase. This requirement is of particular importance

when traffic-actuated signal controls are employed.

The California MUTCD has warrants for the

placement of signals to control vehicular, bicycle and

pedestrian traffic. Pedestrian activated devices,

signals or beacons are not required, but must be

evaluated where directional, multilane, pedestrian

crossings occur. These locations may include:

• Mid-block street crossings;

• Channelized turn lanes;

• Ramp entries and exits; and

• Roundabouts.

The evaluation, selection, programming and use of a

chosen device should be done with guidance from

District Traffic Operations.

403.10 Installation of Traffic Control

Devices

Channelization may provide locations for the

installation of essential traffic control devices, such

as “STOP” and directional signs. See Index 405.4 for

information about the design of traffic islands.

403.11 Summary

• Give preference to the major move(s).


July 2, 2018

• Reduce areas of conflict.

• Reduce the duration of conflicts.

• Cross traffic at right angles or skew no more than

75 degrees. (90 degrees preferred.)

• Separate points of conflict.

• Provide speed-change areas and separate turning

lanes where appropriate.

• Provide adequate width to shadow turning

traffic.

• Restrict undesirable moves with traffic islands.

• Coordinate channelization with effective signal

control.

• Install signs in traffic islands when necessary but

avoid building conflicts one or more modes of

travel.

• Consider all users.

403.12 Other Considerations

• An advantage of curbed islands is they can serve

as pedestrian refuge. Where curbing is

appropriate, consideration should be given to

mountable curbs. See Topic 303 for more

guidance.

• Avoid complex intersections that present

multiple choices of movement to the motorist

and bicyclist.

• Traffic safety should be considered. Collision

records provide a valuable guide to the type of

channelization needed.

Topic 404 - Design Vehicles

404.1 General

Any vehicle, whether car, bus, truck, or recreational

vehicle, while turning a curve, covers a wider path

than the width of the vehicle. The outer front tire can

generally follow a circular curve, but the inner rear

tire will swing in toward the center of the curve.

Some terminology is vital to understanding the

engineering concepts related to design vehicles. See

Index 62.4 Interchanges and Intersection at Grade for

terminology.

404.2 Design Considerations

It may not be necessary to provide for design vehicle

turning movements at all intersections along the State

route if the design vehicle’s route is restricted or it is

not expected to use the cross street frequently.

Discuss with Traffic Operations and the local agency

before a turning movement is not provided. The goal

is to minimize possible conflicts between vehicles,

bicycles, pedestrians, and other users of the roadway,

while providing the minimum curb radii appropriate

for the given situation.

Both the tracking width and swept width should be

considered in the design of roadways for use of the

roadway by design vehicles.

Tracking width lines delineate the path of the vehicle

tires as the vehicle moves through the turn.

Swept width lines delineate the path of the vehicle

body as the vehicle moves through the turn and will

therefore always exceed the tracking width. The

following list of criteria is to be used to determine

whether the roadway can accommodate the design

vehicle.

(1) Traveled way.

(a) To accommodate turn movements(e.g., at

intersections, driveways, alleys, etc.),the

travel way width and intersection design

should be such that tracking width and swept

width lines for the design vehicle do not

cross into any portion of the lane for

opposing traffic. Encroachment into the

shoulder and bike lane is permitted.

(b) Along the portion of roadway where there

are no turning options, vehicles are required

to stay within the lane lines. The tracking

and swept widths lines for the design

vehicle shall stay within the lane as

defined in Index 301.1 and Table 504.3.

This includes no encroachment into Class II

bike lanes.

(2) Shoulders. Both tracking width and swept width

lines may encroach onto paved shoulders to

accommodate turning. For design projects where

the tracking width lines are shown to encroach

onto paved shoulders, the shoulder pavement

structure should be engineered to sustain the

weight of the design vehicle. See Index 613 for

general traffic loading


considerations and Index 626 for tied rigid

shoulder guidance. At corners where no

sidewalks are provided and pedestrians are using

the shoulder, a paved refuge area may be

provided outside the swept width of turning

vehicle.

(3) Curbs and Gutters. Tires may not mount curbs.

If curb and gutter are present and any portion of

the gutter pan is likewise encroached, the gutter

pan must be engineered to match the adjacent

shoulder pavement structure. See Index

613.5(2)(c) for gutter pan design guidance.

(4) Edge of Pavement. To accommodate a turn, the

swept width lines may cross the edge of

pavement provided there are no obstructions.

The tracking width lines must remain on the

pavement structure, including the shoulder,

provided that the shoulder is designed to support

vehicular traffic. If truck volumes are high,

consideration of a wider shoulder is encouraged

in order to preserve the pavement edge.

(5) Bicycle Lanes. Where bicycle lanes are

considered, the design guidance noted above

applies. Vehicles are permitted to cross a bicycle

lane to initiate or complete a turning movement

or for emergency parking on the shoulder. See

the California MUTCD for Class II bike lane

markings.

To accommodate turn movements (e.g.,

intersections, driveways, alleys, etc. are present),

both tracking width and swept width lines may

cross the broken white painted bicycle lane

striping in advance of the right-turn, entering the

bicycle lane when clear to do so.

(6) Sidewalks. Tracking width and swept width lines

must not encroach onto sidewalks or pedestrian

refuge areas, without exception.

(7) Obstacles. Swept width lines may not encroach

upon obstacles including, but not limited to,

curbs, islands, sign structures, traffic

delineators/channelizers, traffic signals, lighting

poles, guardrails, trees, cut slopes, and rock

outcrops.

(8) Appurtenances. Swept width lines do not include

side mirrors or other appurtenances allowed by

the California Vehicle Code, thus,

accommodation to non-motorized users of the

facility and appurtenances should be considered.

If both the tracking width and swept width lines meet

the design guidance listed above, then the geometry

is adequate for that design vehicle. Consideration

should be given to pedestrian crossing distance,

motor vehicle speeds, truck volumes, alignment,

bicycle lane width, sight distance, and the presence

of on-street parking.

Note that the STAA Design Vehicle has a template

with a 56-foot (minimum) and a 67-foot (longer)

radius and the California Legal Design Vehicle has a

template with 50-foot (minimum) and 60-foot

(longer) radii. These templates are shown in Figures

404.5A through 404.5D. The longer radius templates

are more conservative. The longer radius templates

develop less swept width and leave a margin of error

for the truck driver. The longer radius templates

should be used for conditions where the vehicle may

not be required to stop before entering the

intersection.

The minimum radius template can be used if the

longer radius template does not clear all obstacles.

The minimum radius templates demonstrate the

tightest turn that the vehicles can navigate, assuming

a speed of less than 10 miles per hour.

For offtracking lane width requirements on freeway

ramps, see Topic 504.

404.3 Design Tools

District Truck Managers should be consulted early in

the project to ensure compliance with the design

vehicle guidance contained in Topic 404. Consult

local agencies to verify the location of local truck

routes. Essentially, two options are available –

templates or computer software.

• The turning templates in Figures 404.5A through

G are a design aid for determining the swept

width and/or tracking width of large vehicles as

they maneuver through a turn. The templates can

be used as overlays to evaluate the adequacy of

the geometric layout of a curve or intersection

when reproduced on clear film and scaled to

match the highway drawings. These templates

assume a vehicle speed of less than 10 miles per

hour.


December 14, 2018

• Computer software such as AutoTURN or

AutoTrak can draw the swept width and/or

tracking width along any design curve within a

CADD drawing program such as MicroStation or

AutoCAD. Dimensions taken from the vehicle

diagrams in Figures 404.5A through G may be

inputted into the computer program by creating a

custom vehicle if the vehicle is not already

included in the software library. The software

can also create a vehicle turn template that

conforms to any degree curve desired.

404.4 Design Vehicles and Related

Definitions

(1) The Surface Transportation Assistance Act of

1982 (STAA).

(a) STAA Routes. STAA allows certain longer

trucks called STAA trucks to operate on the

National Network. After STAA was

enacted, the Department evaluated State

routes for STAA truck access and created

Terminal Access and Service Access routes

which, together with the National Network,

are called the STAA Network. Terminal

Access routes allow STAA access to

terminals and facilities. Service Access

routes allow STAA trucks one-mile access

off the National Network, but only at

identified exits and only for designated

services. Service Access routes are

primarily local roads. A “Truck Route

Map,” indicating the National Network

routes and the Terminal Access routes is

posted on the Department’s Office of

Commercial Vehicle Operations website

and is also available in printed form.

(b) STAA Design Vehicle. The STAA design

vehicle is a truck tractor-semitrailer

combination with a 48-foot semitrailer, a

43-foot kingpin-to-rear-axle (KPRA)

distance, an 8.5-foot body and axle width,

and a 23-foot truck tractor wheelbase. Note,

a truck tractor is a non-load-carrying

vehicle. There is also a STAA double (truck

tractor-semitrailer-trailer); however, the

double is not used as the design vehicle due

to its shorter turning radius. The STAA

Design Vehicle is shown in Figures 404.5A

and B.

The STAA Design Vehicle in Figures

404.5A or B should be used on the National

Network, Terminal Access, California

Legal, and Advisory routes.

(c) STAA Vehicle – 53-Foot Trailer. Another

category of vehicle allowed only on STAA

routes has a maximum 53-foot trailer, a

maximum 40-foot KPRA for two or more

axles, a maximum 38-foot KPRA for a

single axle, and unlimited overall length.

This vehicle is not to be used as the design

vehicle as it is not the worst case for

offtracking due to its shorter KPRA. The

STAA Design Vehicle should be used

instead.

(2) California Legal.

(a) California Legal Routes. Virtually all State

routes off the STAA Network are California

Legal routes. There are two types of

California Legal routes, the regular

California Legal routes and the KPRA

Advisory Routes. Advisory routes have

signs posted that state the maximum KPRA

length that the route can accommodate

without the vehicle offtracking outside the

lane. KPRA advisories range from 30 feet

to 38 feet, in 2-foot increments. California

Legal vehicles are allowed to use both types

of California Legal routes. California Legal

vehicles can also use the STAA Network.

However, STAA trucks are not allowed on

any California Legal routes. The Truck

Route Map indicating the California Legal

routes is posted on the Department’s Office

of Commercial Vehicle Operations website.

(b) California Legal Design Vehicle. The

California Legal vehicle is a truck tractor-

semitrailer with the following dimensions:

the maximum overall length is 65 feet; the

maximum KPRA distance is 40 feet for

semitrailers with two or more axles, and

38 feet for semitrailers with a single axle;

the maximum width is 8.5 feet. There are

also two categories of California Legal

doubles (truck tractor-semitrailer-trailer);

however, the doubles are not used as the

design vehicle due to their shorter turning

radii. The California Legal Design Vehicle

is shown in Figures 404.5C and D.


The California Legal Design Vehicle in

Figures 404.5C and D should only be used

when the STAA design vehicle is not

feasible and with concurrence from the

District Truck Manager.

(3) 40-Foot Bus.

(a) 40-Foot Bus Routes. All single-unit

vehicles, including buses and motor trucks

up to 40 feet in length, are allowed on

virtually every route in California.

(b) 40-Foot Bus Design Vehicle. The 40-Foot

Bus Design Vehicle shown in Figure

404.5E is an AASHTO standard. Its

25-foot wheelbase and 40-foot length are

typical of city transit buses and some

intercity buses. At intersections where

truck volumes are light or where the

predominate truck traffic consists of mostly

3-axle units, the 40-foot bus may be used.

Its wheel path sweeps a greater width than

3-axle delivery trucks, as well as smaller

buses such as school buses.

(4) 45-Foot Bus & Motorhome.

(a) 45-Foot Bus & Motorhome Routes. The

“45-foot bus and motorhome” refers to bus

and motorhomes over 40 feet in length, up

to and including 45 feet in length. These

longer buses and motorhomes are allowed

in California, but only on certain routes.

The 45-foot tour bus became legal on the

National Network in 1991 and later allowed

on some State routes in 1995. The 45-foot

motorhome became legal in California in

2001, but only on those routes where the 45-

foot bus was already allowed. A Bus and

Motorhome Map indicating where these

longer buses and motorhomes are allowed

and where they are not allowed is posted on

the Department’s Office of Commercial

Vehicle Operations website.

(b) 45-Foot Bus and Motorhome Design

Vehicle. The 45-Foot Bus & Motorhome

Design Vehicle shown in Figure 404.5F is

used by Caltrans for the longest allowable

bus and motorhome. Its wheelbase is

28.5 feet. It is also similar to the AASHTO

standard 45-foot bus. Typically this should

be the smallest design vehicle used on a

State highway. It may be used where the

State highway intersects local streets

without commercial or industrial traffic.

The 45-Foot Bus and Motorhome Design

Vehicle shown in Figure 404.5F should be

used in the design of all interchanges and

intersections on all green routes indicated

on the Bus and Motorhome Map for both

new construction and rehabilitation

projects. Check also the longer standard

design vehicles on these routes as required

– the STAA Design Vehicle and the

California Legal Design Vehicle in Indexes

404.4(1) and (2).

(5) 60-Foot Articulated Bus.

(a) 60-Foot Articulated Bus Routes. The

articulated bus is allowed a length of up to

60 feet per CVC 35400(b)(3)(A). This bus

is used primarily by local transit agencies

for public transportation. There is no

master listing of such routes. Local transit

agencies should be contacted to determine

possible routes within the proposed project.

(b) 60-Foot Articulated Bus Design Vehicle.

The 60-Foot Articulated Bus Design

Vehicle shown in Figure 404.5G is an

AASHTO standard. The routes served by

these buses should be designed to

accommodate the 60-Foot Articulated Bus

Design Vehicle.

404.5 Turning Templates & Vehicle

Diagrams

Figures 404.5A through G are computer-generated

turning templates at an approximate scale of 1"=50' and their associated vehicle diagrams for the design

vehicles described in Index 404.3. The radius of the

template is measured to the outside front wheel path

at the beginning of the curve. Figures 404.5A

through G contain the terms defined as follows:

(1) Tractor Width - Width of tractor body.

(2) Trailer Width - Width of semitrailer body.

(3) Tractor Track - Tractor axle width, measured

from outside face of tires.


December 14, 2018

Figure 404.5A STAA Design Vehicle

56-Foot Radius


Figure 404.5B

STAA Design Vehicle 67-Foot Radius


December 14, 2018

Figure 404.5C

California Legal Design Vehicle 50-Foot Radius


Figure 404.5D

California Legal Design Vehicle 60-Foot Radius


December 14, 2018

Figure 404.5E

40-Foot Bus Design Vehicle


Figure 404.5F

45-Foot Bus & Motorhome Design Vehicle


December 14, 2018

Figure 404.5G

60-Foot Articulated Bus Design Vehicle


(4) Trailer Track – Semitrailer axle width, measured

from outside face of tires.

(5) Lock To Lock Time - The time in seconds that an

average driver would take under normal driving

conditions to turn the steering wheel of a vehicle

from the lock position on one side to the lock

position on the other side. The default in

AutoTurn software is 6 seconds.

(6) Steering Lock Angle - The maximum angle that

the steering wheels can be turned. It is further

defined as the average of the maximum angles

made by the left and right steering wheels with

the longitudinal axis of the vehicle.

(7) Articulating Angle - The maximum angle

between the tractor and semitrailer.

Topic 405 - Intersection Design Standards

405.1 Sight Distance

(1) Stopping Sight Distance. See Index 201.1 for

minimum stopping sight distance requirements.

(2) Corner Sight Distance.

(a) General--At unsignalized intersections a

substantially clear line of sight should be

maintained between the driver of a vehicle,

bicyclist or pedestrian stopped on the minor

road and the driver of an approaching

vehicle on the major road that has no stop.

Line of sight for all users should be included

in right of way, in order to preserve sight

lines.

See DIB 79 for 2R, 3R, certain storm

damage, protective betterment, operational,

and safety projects on two-lane and three-

lane conventional highways.

Adequate time should be provided for the

stopped vehicle on the minor road to either

cross all lanes of through traffic, cross the

near lanes and turn left, or turn right,

without requiring through traffic to

radically alter their speed. The visibility

required for these maneuvers form a clear

sight triangle with the corner sight distance

b and the crossing distance a1 or a2 (see

Figure 405.1 as an example of corner sight

distance at a two-lane, two-way highway).

Dimensions a1 and a2 are measured from the

decision point to the center of the lane. The

actual number of lanes will vary on the

major and minor roads. There should be no

sight obstruction within the clear sight

triangle.

The methodology used for the driver on the

minor road that is stopped to complete the

necessary maneuver while the approaching

vehicle travels at the design speed of the

major road is based on gap-acceptance

behavior. A 7-1/2 second criterion is

applied to a passenger car (including pickup

trucks) for a left turn from a stop on the

minor road. However, this time gap does

not account for a single-unit truck (no

semitrailer), a combination truck (see Index

404.4 for truck tractor-semitrailer

guidance), a right-turn from a stop, or for a

crossing maneuver. See Table 405.1A for

the time gap that addresses these situations

for the assumed design vehicle making

these maneuvers from the minor road.

In determining corner sight distance, a set

back distance for the vehicle waiting on the

minor road must be assumed as measured

from the edge of traveled way of the major

road. Set back for the driver of the vehicle

on the minor road should be a minimum of

10 feet plus the shoulder width of the major

road but not less than 15 feet. The location

of the driver’s eye for the set back is the

decision point per Figure 405.1. Corner

sight distance and the driver’s eye set back

are also illustrated in Figures 405.7 and

504.3I. Line of sight for corner sight

distance for passenger cars is to be

determined from a 3 and 1/2-foot height at

the location of the driver of the vehicle in

the center of the minor road lane to a 3 and

1/2-foot object height in the center of the

approaching outside lane of the major road.

This provides for reciprocal sight by both

vehicles. The passenger car driver’s eye

height should be applied to all minor roads.

In addition, a truck driver’s eye height of

7.6 feet should be applied to the minor road

where applicable. Additionally, if the major

road has a median barrier, a 2-foot object

height should be used to determine the


March 20, 2020

median barrier set back. A median that is

wide enough to accommodate a stopped

vehicle should also provide a clear sight

triangle.

The minimum corner sight distance (feet)

should be determined by the equation:

1.47VmTg, where Vm is the design speed

(mph) of the major road and Tg is the time

gap (seconds) for the minor road vehicle to

enter the major road. The values given in

Table 405.1A should be used to determine

Tg based on the design vehicle, the type of

maneuver, and whether the stopped

vehicle’s rear wheels are on an upgrade

exceeding 3 percent. The distance from the

edge of traveled way to the rear wheels at

the minor road stop location should be

assumed as: 20 feet for a passenger car, 30

feet for a single-unit truck, and 72 feet for a

combination truck.

(b) Public Road Intersections (Refer to

Topic 205 and Index 405.7); corner sight

distance applies, see Table 405.1A.

At signalized intersections the corner sight

distances should also be applied whenever

possible. Even though traffic flows are

designed to move at separate times,

unanticipated conflicts can occur due to

violation of signal, right turns on red,

malfunction of the signal, or use of flashing

red/yellow mode.

The minimum value for corner sight

distance at signalized intersections should

be equal to the stopping sight distance as

given in Table 201.1, measured as

previously described. This includes an

urban driveway that forms a leg of the

signalized intersection.

(c) Private Road Intersections (Refer to

Index 205.2) and Rural Driveways (Refer to

Index 205.4); corner sight distance applies,

see Table 405.1A. If signalized, the

minimum corner sight distance should be

equal to the stopping sight distance as given

in Table 201.1, measured as previously

described.

(d) Urban Driveways (Refer to Index 205.3);

corner sight distance requirements as

described above are not applied to urban

driveways unless signalized. See Index

405.1(2)(b) underlined standard. If parking

is allowed on the major road, parking

should be prohibited on both sides of the

driveway per the California MUTCD,

3B.19.

(3) Decision Sight Distance. At intersections where

the State route turns or crosses another State

route, the decision sight distance values given in

Table 201.7 should be used. In computing and

measuring decision sight distance, the 3.5-foot

eye height and the

0.5-foot object height should be used, the object

being located on the side of the intersection

nearest the approaching driver.

The application of the various sight distance

requirements for the different types of

intersections is summarized in Table 405.1B.

Table 405.1B Application of Sight Distance

Requirements

Intersection

Sight Distance

Types

Stopping

Corner

Decision

Private Roads

X X(1)

Public Streets and

Roads

X X

Signalized

Intersections

X X(2)

State Route Inter-

sections & Route

Direction

Changes, with or

without Signals

X X X

NOTES:

(1) Per Index 405.1(2)(c), the minimum corner sight

distance shall be equal to the stopping sight

distance as given in Table 201.1. See Index

405.1(2)(a) for setback requirements.

(2) Apply corner sight distance requirements at

signalized intersections whenever possible due to

unanticipated violations of the signals or

malfunctions of the signals. See Index 405.1(2)(b).


Figure 405.1

Corner Sight Distance (b)

Table 405.1A

Corner Sight Distance Time Gap (Tg)

for Unsignalized Intersections

Design Vehicle Left-turn from Stop (s) (4) Right-turn from Stop and

Crossing Maneuver (s)

Passenger Car 7½ 6½

Private Road Intersection

Rural Driveway

Single-Unit Truck 9½ 8½

Public Road Intersection

Combination Truck 11½ 10½

Major and Minor Roads on Routes:

National Network

Terminal or Service Access

California Legal

KPRA Advisory

Notes: Time gaps are for a stopped vehicle to turn left, right or cross a two-lane highway with no median and with minor

road grades of 3 percent or less. The table values should be adjusted as follows:

(1) For multilane highways—When crossing or making a left-turn onto a two-way major road with more than two lanes,

add 0.5 s for passenger cars or 0.7 s for trucks for each additional lane to be crossed. Median widths should be converted to

an equivalent number of lanes in applying the 0.5 s and 0.7 s criteria. For example, an 18-foot wide median is equivalent to

1.5 lanes; this requires an additional 0.75 s for a passenger car to cross or an additional 1.05 s for a truck to cross.

(2) For minor road approach grades—If the minor road approach grade is an upgrade that exceeds 3 percent and the rear

wheels of the design vehicle are on the grade exceeding 3 percent, add 0.2 s for each percent grade for left-turns and crossing

maneuvers; or add 0.1 s for each percent grade for right-turns. For example, a passenger car is turning right from a minor

road and at the stop location its rear wheels are on a 4 percent upgrade; this requires an additional 0.4 s for the right-turn.

(3) Unique situations may necessitate a different design vehicle for a particular minor road than those listed here (e.g.,

predominant combination trucks out of a rural driveway). Additionally, for intersections at skewed angles less than 60

degrees, a further adjustment is needed. See the AASHTO “A Policy on Geometric Design of Highways and Streets” for

guidance.

(4) Time gap for vehicles approaching from the left can be the same as the right-turn from stop maneuver.


March 20, 2020

(4) Acceleration Lanes for Turning Moves onto

State Highways. At rural intersections, with

“STOP” control on the local cross road,

acceleration lanes for left and right turns onto

the State facility should be considered. At a

minimum, the following features should be

evaluated for both the major highway and the

cross road:

• divided versus undivided

• number of lanes

• design speed

• gradient

• lane, shoulder and median width

• traffic volume and composition of highway

users, including trucks and transit vehicles

• turning volumes

• horizontal curve radii

• sight distance

• proximity of adjacent intersections

• types of adjacent intersections

For additional information and guidance,

refer to AASHTO, A Policy on Geometric

Design of Highways and Streets, the

District Traffic Engineer or designee, the

District Design Liaison, and the Project

Delivery Coordinator.

405.2 Left-turn Channelization

(1) General. The purpose of a left-turn lane is to

expedite the movement of through traffic by,

controlling the movement of turning traffic,

increasing the capacity of the intersection, and

improving safety characteristics.

The District Traffic Branch normally

establishes the need for left-turn lanes.

(2) Design Elements.

(a) Lane Width – The lane width for both

single and double left-turn lanes on State

highways shall be 12 feet.

For conventional State highways with

posted speeds less than or equal to

40 miles per hour and AADTT (truck

volume) less than 250 per lane that are in

urban, city or town centers (rural main

streets), the minimum lane width shall be

11 feet.

When considering lane width reductions

adjacent to curbed medians, refer to Index

303.5 for guidance on effective roadway

width, which may vary depending on

drivers’ lateral positioning and shy distance

from raised curbs.

(b) Approach Taper -- On conventional

highways without a median, an approach

taper provides space for a left-turn lane by

moving traffic laterally to the right. The

approach taper is unnecessary where a

median is available for the full width of the

left-turn lane. Length of the approach taper

is given by the formula on

Figures 405.2A, B and C.

Figure 405.2A shows a standard left-turn

channelization design in which all widening

is to the right of approaching traffic and the

deceleration lane (see below) begins at the

end of the approach taper. This design

should be used in all situations where space

is available, usually in rural and semi-rural

areas or in urban areas with high traffic

speeds and/or volumes.

Figures 405.2B and 405.2C show alternate

designs foreshortened with the deceleration

lane beginning at the 2/3 point of the

approach taper so that part of the

deceleration takes place in the through

traffic lane. Figure 405.2C is shortened

further by widening half (or other

appropriate fraction) on each side. These

designs may be used in urban areas where

constraints exist, speeds are moderate and

traffic volumes are relatively low.

(c) Bay Taper -- A reversing curve along the

left edge of the traveled way directs traffic

into the left-turn lane. The length of this

bay taper should be short to clearly delin-

eate the left-turn move and to discourage

through traffic from drifting into the left-

turn lane. Table 405.2A gives offset data

for design of bay tapers. In urban areas,

lengths of 60 feet and 90 feet are normally


used. Where space is restricted and speeds

are low, a 60-foot bay taper is appropriate.

On rural high-speed highways, a 120-foot

length is considered appropriate.

(d) Deceleration Lane Length -- Design speed

of the roadway approaching the intersection

should be the basis for determining

deceleration lane length. It is desirable that

deceleration take place entirely off the

through traffic lanes. Deceleration lane

lengths are given in Table 405.2B; the bay

taper length is included. Where partial

deceleration is permitted on the through

lanes, as in Figures 405.2B and 405.2C,

design speeds in Table 405.2B may be

reduced

10 miles per hour to 20 miles per hour for a

lower entry speed. In urban areas where

cross streets are closely spaced and

deceleration lengths cannot be achieved, the

District Traffic branch should be consulted

for guidance.

(e) Storage Length -- At unsignalized inter-

sections, storage length may be based on the

number of turning vehicles likely to arrive

in an average 2-minute period during the

peak hour. At a minimum, space for 2

vehicles should be provided at 25 feet per

vehicle. If the peak hour truck traffic is 10

percent or more, space for at least one

passenger car and one truck should be

provided. Bus usage may require a longer

storage length and should be evaluated if

their use is anticipated.

At signalized intersections, the storage

length may be based on one and one-half to

two times the average number of vehicles

that would store per signal cycle depending

on cycle length, signal phasing, and arrival

and departure rates. At a minimum, storage

length should be calculated in the same

manner as unsignalized intersection. The

District Traffic Branch should be consulted

for this information.

When determining storage length, the end

of the left-turn lane is typically placed at

least 3 feet, but not more than 30 feet, from

the nearest edge of shoulder of the

intersecting roadway. Although often set by

the placement of a crosswalk line or limit

line, the end of the storage lane should

always be located so that the appropriate

turning template can be accommodated.

Table 405.2A Bay Taper for Median Speed-change Lanes

NOTES:

(1) The table gives offsets from a base line parallel to the

edge of traveled way at intervals measured from point

"A". Add "E" for measurements from edge of traveled

way.

(2) Where edge of traveled way is a curve, neither base

line nor taper between B & C will be a tangent. Use

proportional offsets from B to C.

(3) The offset "E" is usually 2 ft along edge of traveled

way for curbed medians; Use "E" = 0 ft. for striped

medians.

Table 405.2B Deceleration Lane Length

Design Speed

(mph)

Length to

Stop (ft)

30 235

40 315

50 435

60 530


December 14, 2018

(3) Double Left-turn Lanes. At signalized

intersections on multilane conventional

highways and on multilane ramp terminals,

double left-turn lanes should be considered if

the left-turn demand is 300 vehicles per hour or

more. The lane widths and other design

elements of left-turn lanes given under

Index 405.2(2) applies to double as well as

single left-turn lanes.

The design of double left-turn lanes can be

accomplished by adding one or two lanes in the

median. See "Complete Intersections: A Guide

to Reconstructing Intersections and

Interchanges for Bicyclists and Pedestrians",

published by Headquarters, Division of Traffic

Operations, for the various treatments of double

left-turn lanes.

(4) Two-way Left-turn Lane (TWLTL). The

TWLTL consists of a striped lane in the median

of an arterial and is devised to address the

special capacity and safety problems associated

with high-density strip development. It can be

used on 2-lane highways as well as multilane

highways. Normally, the District Traffic

Operations Branch should determine the need

for a TWLTL.

The minimum width for a TWLTL shall be

12 feet (see Index 301.1). The preferred width

is 14 feet. Wider TWLTL's are occasionally

provided to conform with local agency

standards. However, TWLTL's wider than

14 feet are not recommended, and in no case

should the width of a TWLTL exceed 16 feet.

Additional width may encourage drivers in

opposite directions to use the TWLTL

simultaneously.

405.3 Right-turn Channelization

(1) General. For right-turning traffic, delays are

less critical and conflicts less severe than for

left-turning traffic. Nevertheless, right-turn

lanes can be justified on the basis of capacity,

analysis, and crash experience.

In rural areas a history of high speed rear-end

collisions may warrant the addition of a right-

turn lane.

In urban areas other factors may contribute to

the need such as:

• High volumes of right-turning traffic

causing backup and delay on the through

lanes.

• Conflicts between crossing pedestrians and

right-turning vehicles and bicycles.

• Frequent rear-end and sideswipe collisions

involving right-turning vehicles.

Where right-turn channelization is proposed,

lower speed right-turn lanes should be provided

to reduce the likelihood of conflicts between

vehicles, pedestrians, and bicyclists.

(2) Design Elements.

(a) Lane and Shoulder Width--Index 301.1

shall be used for right-turn lane width

requirements. Shoulder width shall be a

minimum of 4 feet. Although not

desirable, lane and shoulder widths less than

those given above can be considered for

right-turn lanes under the following

conditions pursuant to Index 82.2:

• In urban, city or town centers (rural

main streets) with posted speeds less

than 40 miles per hour in severely

constrained situations, if truck or bus

use is low, consideration may be given

to reducing the right-turn lane width to

10 feet.

• Shoulder widths may also be

considered for reduction under

constricted situations. Whenever

possible, at least a 2-foot shoulder

should be provided where the right-turn

lane is adjacent to a curb. Entire

omission of the shoulder should only be

considered in constrained situations and

where an 11-foot lane can be

constructed.

Gutter pans can be included within a

shoulder, but cannot be included as part

of the travel lane width. Additional

right of way for a future right-turn lane

should be considered when an

intersection is being designed.


Figure 405.2A Standard Left-turn Channelization


December 14, 2018

Figure 405.2B Minimum Median Left-turn Channelization

(Widening on one Side of Highway)


Figure 405.2C Minimum Median Left-turn Channelization

(Widening on Both Sides in Urban Areas with Short Blocks)


December 14, 2018

(b) Curve Radius--Where pedestrians are

allowed to cross a free right-turning

roadway, the curve radius should be such

that the operating speed of vehicular traffic

is no more than 20 miles per hour at the

pedestrian crossing. See NCHRP Report

672, “Roundabouts: An Informational

Guide” for guidance on the determination of

design speed (fastest path) for turning

vehicles. See Index 504.3(3) for additional

information.

(c) Tapers--Approach tapers are usually un-

necessary since main line traffic need not be

shifted laterally to provide space for the

right-turn lane. If, in some rare instances, a

lateral shift were needed, the approach taper

would use the same formula as for a left-

turn lane.

Bay tapers are treated as a mirror image of

the left-turn bay taper.

(d) Deceleration Lane Length--The conditions

and principles of left-turn lane deceleration

apply to right-turn deceleration. Where full

deceleration is desired off the high-speed

through lanes, the lengths in Table 405.2B

should be used. Where partial deceleration

is permitted on the through lanes because of

limited right of way or other constraints,

average running speeds in Table 405.2B

may be reduced 10 miles per hour to

20 miles per hour for a lower entry speed.

For example, if the main line speed is

50 miles per hour and a 10 miles per hour

deceleration is permitted on the through

lanes, the deceleration length may be that

required for 40 miles per hour.

(e) Storage Length--Right-turn storage length

is determined in the same manner as left-

turn storage length. See Index 405.2(2)(e).

(3) Right-turn Lanes at Off-ramp Intersections.

Diamond off-ramps with a free right-turn at the

local street and separate right-turn off-ramps

around the outside of a loop will likely cause

conflict as traffic volumes increase. Serious

conflicts occur when the right-turning vehicle

must weave across multiple lanes on the local

street in order to turn left at a major cross street

close to the ramp terminal. Furthermore, free

right-turns create sight distance issues for

pedestrians and bicyclists crossing the off-ramp,

or pedestrians crossing the local road. Also,

rear-end collisions can occur as right-turning

drivers slow down or stop waiting for a gap in

local street traffic. Free right-turns usually end

up with ”YIELD”, ”STOP”, or signal controls

thus defeating their purpose of increasing

intersection capacity.

405.4 Traffic Islands

A traffic island is an area between traffic lanes for

channelization of bicycle and vehicle movements or

for pedestrian refuge. An island may be defined by

paint, raised pavement markers, curbs, pavement

edge, or other devices. The California MUTCD

should be referenced when considering the

placement of traffic islands at signalized and

unsignalized locations. For splitter island guidance at

roundabouts, see Index 405.10(13).

Traffic islands usually serve more than one function.

These functions may be:

(a) Channelization to confine specific traffic

movements into definite channels;

(b) Divisional to separate traffic moving in the same

or opposite direction; and

(c) Refuge, to aid users crossing the roadway.

Generally, islands should present the least potential

conflict to approaching or crossing bicycles and

vehicles, and yet perform their intended function.

(1) Design of Traffic Islands. Island sizes and

shapes vary from one intersection to another.

They should be large enough to command

attention. Channelizing islands should not be

less than 50 square feet in area, preferably

75 square feet. Curbed, elongated divisional

median islands should not be less than 4 feet

wide and 20 feet long. All traffic islands placed

in the path of a pedestrian crossing must comply

with DIB 82. See the Standard Plans for typical

island passageway details.

The approach end of each island should be

offset 3 feet to the left and 5 feet to the right of

approaching traffic, using standard 1:15

parabolic flares, and clearly delineated so that it

does not surprise the motorist or bicyclist.

These offsets are in addition to the shoulder


widths shown in Table 302.1. Table 405.4 gives

standard parabolic flares to be used in island

design. On curved alignment, parabolic flares

may be omitted for small triangular traffic

islands whose sides are less than 25 feet long.

The approach nose of a divisional island should

be highly visible day and night with appropriate

use of signs (reflectorized or illuminated) and

object markers. The approach nose should be

offset 3 feet from the through traffic to minimize

accidental impacts.

(2) Delineation of Traffic Islands. Generally,

islands should present the least potential

conflict to approaching traffic and yet perform

their intended function. See Index 303.2 for

appropriate curb type. Islands may be

designated as follows:

(a) Raised paved areas outlined by curbs.

(b) Flush paved areas outlined by pavement

markings.

(c) Unpaved areas (small unpaved areas should

be avoided).

On facilities with posted speeds over 40 miles

per hour, the use of any type of curb is

discouraged. Where curbs are to be used, they

should be located at or outside of the shoulder

edge, as discussed in Index 303.5.

In rural areas, painted channelization sup-

plemented with raised pavement markers may

be more appropriate than a raised curbed

channelization. This design is as forgiving as

possible and decreases the consequence of a

driver's or bicyclist’s failure to detect or

recognize the curbed island. Consideration for

snow removal operations should be determined

where appropriate.

In urban areas, posted speeds less than or equal

to 40 miles per hour allow more frequent use of

curbed islands. Local agency requirements and

matching existing conditions are factors to

consider.

(3) Pedestrian Refuge

Pedestrian refuge islands allow pedestrians to

cross fewer lanes at a time while judging

conflicts separately. They also provide a refuge

so slower pedestrians can wait for a gap in

traffic while reducing total crossing distance.

At unsignalized intersections in rural city/town

centers (rural main streets), suburban, or urban

areas, a pedestrian refuge should be provided

between opposing traffic where pedestrians are

allowed to cross 2 or more through traffic lanes

in one direction of travel, at marked or

unmarked crosswalks. Pedestrian islands at

signalized crosswalks should be considered,

taking into account crossing distance and

pedestrian activity. Note that signalized

pedestrian crossings must be timed to allow for

pedestrians to cross. See the California

MUTCD, Chapter 4E, for further guidance.

Traffic islands used as pedestrian refuge are to

be large enough to provide a minimum of

6 feet in the direction of pedestrian travel,

without exception.

All traffic islands placed in the path of a

pedestrian crossing must be accessible, refer to

DIB 82 and the Standard Plans for further

guidance. An example of a traffic island that

serves as a pedestrian refuge is shown on Figure

405.4.

405.5 Median Openings

(1) General. Median openings, sometimes called

crossovers, provide for crossings of the median

at designated locations. Except for emergency

passageways in a median barrier, median

openings are not allowed on urban freeways.

Median openings on expressways or divided

conventional highways should not be curbed

except when the median between openings is

curbed, or it is necessary for delineation of

traffic signal standards and other necessary

hardware, or for protection of pedestrians. In

these special cases B4 curbs should be used. An

example of a median opening design is shown

on Figure 405.5.

(2) Spacing and Location. By a combination of

interchange ramps and emergency

passageways, provisions for access to the

opposite side of a freeway may be provided for

law enforcement, emergency, and maintenance

vehicles to avoid extreme out-of-direction

travel. Access should not be more frequent


December 14, 2018

Table 405.4

Parabolic Curb Flares Commonly Used

OFFSET IN FEET FOR GIVEN "X" DISTANCE

Distance

Length

of Flare

L X

10 15 20 25 30 40 45 50 60 70 75 80 90 100 110 120

1:5 FLARES

25 0

.

8

0

0.80 1.80 3.20 5.00

50 0

.

0

8

0.40 1.60 3.60 6.40 10.00

1:10 FLARES

50 0

.

2

0

0.20 0.80 1.80 3.20 5.00

100 0.10 0.40

0.90 1.60 2.50 3.60 4.90 6.40 8.10 10.00

1:15 FLARES

45 0

.

1

5

0.15 0.59 1.33 2.37 3.00

75 0

.

0

9

0.09 0.36 0.80 1.42 2.22 3.20 4.36 5.00

90 0.07 0.30 0.67 1.19 1.85 2.67 3.63 4.74 6.00

120 0

.

0

7

0.06 0.22 0.50 0.89 1.39 2.00 2.72 3.56 4.50 5.56 6.72 8.00


Figure 405.4

Pedestrian Refuge Island

than at three-mile intervals. See Traffic Safety

Systems Guidance for additional information

on the design of emergency passageways.

Emergency passageways should be located

only where decision sight distance is available

(see Table 201.7).

Median openings at close intervals on other

types of highways create conflicts with high

speed through traffic. Median openings should

be spaced at intervals no closer than 1600 feet.

If a median opening falls within 300 feet of an

access opening, it should be placed opposite the

access opening.

(3) Length of Median Opening. For any three or

four-leg intersection on a divided highway, the

length of the median opening should be at least

as great as the width of the crossroads

pavement, median width, and shoulders. An

important factor in designing median openings

is the path of the design vehicle making a

minimum left turn at 5 miles per hour to

10 miles per hour. The length of median

opening varies with width of median and angle

of intersecting road.

Usually a median opening of 60 feet is

adequate for 90 degree intersections with

median widths of 22 feet or greater. When the

median width is less than 22 feet, a median

opening of 70 feet is needed. When the

intersection angle is other than 90 degrees, the

length of median opening should be established

by using truck turn templates (see Index 404.3).

(4) Cross Slope. The cross slope in the median

opening should be limited to 5 percent.

Crossovers on curves with super elevation

exceeding 5 percent should be avoided. This

cross slope may be exceeded when an existing

2-lane roadbed is converted to a 4-lane divided

highway. The elevation of the new construction

should be based on the 5 percent cross slope

requirement when the existing roadbed is

raised to its ultimate elevation.

(5) References. For information related to the

design of intersections and median openings,

"A Policy on Geometric Design of Highways

and Streets," AASHTO, should be consulted.

405.6 Access Control

The basic guidance which govern the extent to

which access rights are to be acquired at

interchanges (see Topic 104, Index 205.1 and 504.8

and the PDPM) also apply to intersections at grade

on expressways. Cases of access control which

frequently occur at intersections are shown in Figure

405.7. This illustration does not presume to cover all

situations. Where required by traffic conditions,

access should be extended in order to ensure proper

operation of the expressway lanes. Reasonable

variations which observe the basic principles

referred to above are acceptable.

However, negative impacts on the mobility needs of

pedestrians, bicyclists, equestrians, and transit users

need to be assessed. Pedestrians and bicyclists are

sensitive to additional out of direction travel.


December 14, 2018

Figure 405.5

Typical Design for Median Openings


405.7 Public Road Intersections

The basic design to be used at right-angle public

road intersections on the State Highway System is

shown in Figure 405.7. The essential elements are

sight distance (see Index 405.1) and the treatment of

the right-turn on and off the main highway.

Encroachment into opposing traffic lanes by the

turning vehicle should be avoided or minimized.

(1) Right-turn Onto the Main Highway. The

combination of a circular curve joined by a 2:1

taper on the crossroads and a 75-foot taper on

the main highway is designed to fit the wheel

paths of the appropriate turning template

chosen by the designer.

It is desirable to keep the right-turn as tight as

practical, so the “STOP” or “YIELD” sign on

the minor leg can be placed close to the inter-

section.

(2) Right-turn Off the Main Highway. The

combination of a circular curve joined by a

150-foot taper on the main highway and a

4:1 taper on the crossroads is designed to fit the

wheel paths of the appropriate turning template

and to move the rear of the vehicle off the main

highway. Deceleration and storage lanes may

be provided when necessary (see Index 405.3).

(3) Alternate Designs. Offsets are given in Figure

405.7 for right angle intersections. For skew

angles, roadway curvature, and possibly other

reasons, variations to the right-angle design are

permitted, but the basic rule is still to

approximate the wheel paths of the design

vehicle.

A three-center curve is an alternate treatment

that may be used at the discretion of the

designer.

Intersections are major consideration in bicycle

path design as well. See Indexes 403.6 and

1003.1(5) for general bicycle path intersection

design guidance. Also see Section 5.3 of the

AASHTO Guide for the Planning, Design, and

Operation of Bicycle Facilities.

405.8 City Street Returns and Corner Radii

The pavement width and corner radius at city street

intersections is determined by the type of vehicle to

be accommodated and the mobility needs of

pedestrians and bicyclists, taking into consideration

the amount of available right of way, the types of

adjoining land uses, the place types, the roadway

width, and the number of lanes on the intersecting

street.

At urban intersections, the California truck or the

Bus Design Vehicle template may be used to

determine the corner radius. Where STAA truck

access is allowed, the STAA Design Vehicle

template should be used giving consideration to

factors mentioned above. See Index 404.3.

Smaller radii of 15 feet to 25 feet are appropriate at

minor cross streets where few trucks or buses are

turning. Local agency standards may be appropriate

in urban and suburban areas.

Encroachment into opposing traffic lanes must be

avoided.

405.9 Widening of 2-lane Roads at

Signalized Intersections

Two-lane State highways may be widened at

intersections to 4-lanes whenever signals are

installed. Sometimes it may be necessary to widen

the intersecting road. The minimum design is shown

in Figure 405.9. More elaborate treatment may be

warranted by the volume and pattern of traffic

movements. Unusual turning movement patterns

may possibly call for a different shape of widening.

The impact on pedestrian and bicycle traffic

mobility of larger intersections should be assessed

before a decision is made to widen an intersection.

405.10 Roundabouts

Roundabout intersections on the State highway

system must be developed and evaluated in

accordance with National Cooperative Highway

Research Program (NCHRP) Report 672 entitled

“Roundabouts: An Informational Guide, 2nd ed.”

(NCHRP Guide 2) dated October 2010 and Traffic

Operations Policy Directive (TOPD) Number

13-02. Also see Index 401.5 for general information

and guidance. See Figure 405.10 Roundabout

Geometric Elements for nomenclature associated

with roundabouts. Signs, striping and markings at

roundabouts are to comply with the California

MUTCD.


December 14, 2018

Figure 405.7 Public Road Intersections


Figure 405.9 Widening of Two-lane Roads at Signalized Intersections


December 14, 2018

A roundabout is a form of circular intersection in

which traffic travels counterclockwise around a

central island and entering traffic must yield to the

circulating traffic. Roundabouts feature, among

other things, a central island, a circulatory roadway,

and splitter islands on each approach. Roundabouts

rely upon two basic and important operating

principles:

(a) Speed reduction at the entry and through the

intersection will be achieved through geometric

design and,

(b) The yield-at-entry rule, which requires traffic

entering the intersection to yield to traffic that is

traveling in the circulatory roadway.

Benefits of roundabouts are:

• Fewer conflict points typically result in fewer

collisions with less severity. Over half of vehicle

to vehicle points of conflict associated with

intersections are eliminated with the use of a

roundabout. Additionally, a roundabout

separates the points of conflict which eases the

ability of the users to identify a conflict and

helps prevent conflicts from becoming

collisions.

• Roundabouts are designed to reduce the

vehicular speeds at intersections. Lower speeds

lessens the vehicular collision severity.

Likewise, studies indicate that pedestrian and

bicyclist collisions with motorized vehicles at

lower speeds significantly reduce their severity.

• Roundabouts allow continuous free flow of

vehicles and bicycles when no conflicts exist.

This results in less noise and air pollution and

reduces overall delays at roundabout

intersections.

Except as indicated in this Index, the standards

elsewhere in this manual do not apply to

roundabouts. For the application of design

standards, the approach ends of the splitter islands

define the boundary of a roundabout intersection,

see Figure 405.10. The design standards elsewhere

in this manual apply to the approach legs beyond the

approach ends of the splitter islands.

(1) Design Period.

First consider the design of a single lane

roundabout per the design period guidance in

Index 103.2. If a second lane is not needed

until 10 or more years, it may be better to phase

the improvements. Construct the first phase of

the roundabout so at the 20-year design period,

an additional lane can be easily added. In order

to comply with the 20-year design period, the

initial project must provide the right of way

needed for utility relocations, a shared-use path

designed for a Class I Bikeway, and all other

features other than pavement, lighting, and

striping in their ultimate locations.

In some locations, it may not be practical to

build a single lane roundabout that will operate

for 10 years. Geometric constraints and other

conflicts may preclude widening to the ultimate

configuration. In such cases, other intersection

configurations or control strategies addressed

in Index 401.5 may need to be considered.

When staging improvements, see NCHRP

Guide 2, Section 6.12.

(2) Design Vehicles - See Topic 404.

The turning path for the design vehicle, see

Index 404.5, dictates many of the roundabout

dimensions. The design vehicle tracking and

swept width are to be used when designing all

the entries and exits, where design vehicles are

unrestricted (see Index 404.2), and the

circulatory roadway. The percentage of trucks

and their lane utilization is an important

consideration on multilane roundabouts when

determining if the design will allow trucks to

stay within their own lane or encroach into the

adjacent lane. If permit vehicles larger than the

design vehicle occasionally use the proposed

roundabout, they can be accommodated by

having removable signs or other removable

features in the central island or around the

circular path to ensure their swept path can

negotiate the roundabout. Roundabouts should

not be overdesigned for the occasional permit

vehicle.

To accurately simulate the design vehicle

swept width traveling through a roundabout,

the minimum speed of the design vehicle used

in computer simulation software (e.g., Auto


TURN) should be 10 miles per hour through

the roundabout.

(3) Inscribed Circle Diameter.

At single lane roundabouts, the size of the

inscribed circle is largely dependent upon the

turning requirements of the design vehicle. The

inscribed circle diameter (ICD) must be large

enough to accommodate: (a) the STAA design

vehicle for all roundabouts on the National

Network and on Terminal Access routes; and,

(b) the California Legal design vehicle on all

non-STAA route intersections on California

Legal routes and California Legal KPRA

Advisory routes, while maintaining adequate

deflection curvature to ensure appropriate

travel speeds for smaller vehicles. The design

vehicle is to navigate the roundabout with the

front tractor wheels off the truck apron, if one

is present. Transit vehicles, fire engines and

single-unit delivery vehicles are also to be able

to navigate the roundabout without using the

truck apron, if one is present. The inscribed

circle diameter for a single lane roundabout

generally ranges between 105 feet to 150 feet

to accommodate the California Legal design

vehicle and 130 feet to 180 feet to

accommodate the STAA design vehicle.

At multilane roundabouts, the inscribed circle

diameter is to achieve adequate alignment of

the natural vehicle path while maintaining

deflection curvature to ensure appropriate

travel speeds. To achieve both of these design

objectives requires a slightly larger diameter

than used for a single lane roundabout. The

inscribed circle diameter for a multilane

(2-lane) roundabout generally ranges between

150 feet to 220 feet to accommodate the

California Legal design vehicle for non-STAA

route intersections on California Legal routes

and California Legal KPRA Advisory routes,

and 165 feet to 220 feet to accommodate the

STAA design vehicle for roundabouts on the

National Network and on Terminal Access

routes. Similar to a single lane roundabout, the

design vehicle is to be able to navigate a

multilane roundabout with the front tractor

wheels staying off the truck apron, if one is

present. Transit vehicles, fire engines and

single-unit delivery vehicles are also to be able

to navigate the roundabout without using the

truck apron, if one is present.

The inscribed diameter ranges given above are

typical values, design may be larger or smaller.

Site location constraints and performance

checks will determine if the diameter is

appropriate for the location.

(4) Entry Speeds.

Lowering the speed of vehicles entering and

traveling through the roundabout is a primary

design objective that is achieved by approach

alignment and entry geometry.

The following entry speeds should not be

exceeded:

• Single lane entry, 25 miles per hour.

• Multilane entry, 30 miles per hour.

A bypass lane is not included in the number of

entry lanes. A bypass prohibits entry into the

circulatory roadway.

Entry speeds are to be determined through

fastest path analysis. Fastest path is the

smoothest, flattest path possible for a single

vehicle in the absence of other traffic and

ignoring all lane markings. The fastest path

analysis should begin at least 165 feet from the

inscribed circle diameter and should not bring

the path closer than 3 feet from a stripe nor

5 feet from the face of a curb. These distances

are minimums and the fastest path may occur

further away from the curbs and striping

depending on the roundabout configuration.

For fastest path evaluation, see NCHRP Guide

2, Section 6.7.1.

(5) Exit Design.

Similar to entry design, exit design flexibility

is required to achieve the optimal balance

between competing design variables and

project objectives to provide adequate capacity

and, essentially, safety while minimizing

excessive property impacts and costs. Thus,

the selection of a curved versus tangential

design is to be based upon the balance of

each of these criteria. Exit design is

influenced by the place type, pedestrian

demand, bicyclist needs, the design vehicle


December 14, 2018

and physical constraints. The exit curb radii

are usually larger than the entry curb radii in

order to minimize the likelihood of congestion

and crashes at the exits. However, the desire to

minimize congestion at the exits needs to be

balanced with the need to maintain an

appropriate operating speed through the

pedestrian crossing. Therefore, the exit path

radius should not be significantly greater than

the circulating path radius to ensure low speeds

are maintained at the pedestrian crossing.

(6) Number of Legs Serving the Roundabout.

Intersections with more than four legs are often

difficult to manage operationally. Roundabouts

are a proven traffic control device in such

situations. However, it is necessary to ensure

that the design vehicle can maneuver through

all unrestricted legs of the roundabout.

(7) Pedestrian Use.

Sidewalks around the circular roadway are to

be designed as shared-use paths, see

Index 405.10(8)(c). However, the guidance in

Design Information Bulletin (DIB) 82

Pedestrian Accessibility Guidelines for

Highway Projects must also be followed when

designing these shared-use facilities around a

roundabout. If there is a difference in the

standards, the guidance in DIB 82 is to be

followed. In addition,

(a) Pedestrian curb ramps need to be

differentiated from bike ramps:

• The detectable warning surface

(truncated domes) differentiates a

pedestrian curb ramp from a bicycle

ramp.

• Detectable warning surface is required

on curb ramps. They are not to be used

on a bike ramp.

(b) Truck aprons and mountable curbs are not

to be placed in the pedestrian crossing

areas.

(c) See the California MUTCD for the signs

and markings used at roundabouts.

(d) At pedestrian crossing locations the

accessibility design will be treated as a

midblock pedestrian street crossing. See

DIB 82 for more information.

(8) Bicyclist Use.

(a) General. Bicyclists may choose to travel in

the circular roadway of a roundabout by

taking a lane, while others may decide to

travel using the shared-use path to bypass

the circular roadway. Therefore, the

approach and circular roadways, as well as

the shared-use path all need to be designed

for the mobility needs of bicyclists. See the

California MUTCD for the signs and

markings used at roundabouts.

(b) Bicyclist Use of the Circular Roadway.

Single lane roundabouts do not require

bicyclists to change lanes in the circular

roadway to select the appropriate lane for

their direction of travel, so they tend to be

comfortable for bicyclists to use. Even

two-lane roundabouts, which may have

straighter paths of travel that can lead to

faster vehicular traveling speeds, appear to

be comfortable for bicyclists that prefer to

travel like vehicles. Roundabouts that have

more than two circular lanes can create

complexities in signing and striping (see

the California MUTCD for guidance), and

their operating speed may cause some

bicyclists to decide to bypass the circular

roadway and use the bicycle ramp that

provides access to the shared-use path

around the roundabout.

(c) Bicyclists Use of the Shared-Use Path.

The shared-use path is to be designed using

the guidance in Index 1003.1 for Class I

Bikeways and in NCHRP Guide 2

Section 6.8.2.2. However, the accessibility

guidance in DIB 82 must also be followed

when designing these shared-use facilities

around a roundabout. If there is a

difference in the standards, the

accessibility guidance in DIB 82 is to be

followed to ensure the facility is accessible

to pedestrians with disabilities.

Bicycle ramps are to be located to avoid

confusion as curb ramps for pedestrians.

Also see Index 405.10(7) for guidance on

how to differentiate the two types of ramps.


The design details and width of the ramp

are also important to the bicyclist.

Bicyclists approaching the bicycle ramp

need to be provided the choice of merging

left into the lane or moving right to use the

bicycle ramp. Bicycle ramps should be

placed at a 35 to 45 degree angle to the

departure roadway and the sidewalk to

enable the bicyclists to use the ramp and

discourage bicyclists from entering the

shared-use path at a speed that is

detrimental to the pedestrians. The shared-

use path should be designated as Class I

Bikeways; however, appropriate

regulatory signs may need to be posted if

the local jurisdiction has a law(s) that

prohibit bicyclists from riding on a

sidewalk.

A landscape buffer or strip between the

shared-use/Class I Bikeway and the

circular roadway of the roundabout is

needed and should be a minimum of 2 feet

wide.

Pedestrian crossings may also be used by

bicyclists; thus, these shared-use crossings

need to be designed for both bicyclist and

pedestrian needs.

(9) Transit Use.

Transit vehicles and buses will not have

difficulty negotiating a roundabout when it has

been designed using the California Legal

design vehicle or the STAA design vehicle.

However, to minimize passenger discomfort, a

roundabout should be designed such that the

transit vehicle or bus does not use the truck

apron, if one is present.

(10) Stopping Sight Distance and Visibility.

See Index 201.1 for stopping sight distance

guidance at roundabouts.

A domed or mounded central island, between

3.5 to 6 feet high, is needed to focus attention

on the approach and through roundabout

alignment. A domed central island provides a

visual screen from downstream alignment and

other distractions and provides a visual cue for

vehicles approaching the roundabout.

In high speed environments, additional lighting

of, and vertical elements in the central island

(i.e., landscaping and esthetic features) may be

needed.

(11) Speed Consistency.

Consistency in operating speeds between the

various movements within the roundabout can

minimize collisions between traffic streams.

The operating speeds between competing

traffic streams and between consecutive

geometric elements should be minimized such

that the maximum speed differential between

them is no more than 15 miles per hour; it is

preferred that the operating speed differential

be less than 10 miles per hour.

(12) Path Alignment (Natural Path).

As two traffic streams approach the roundabout

in adjacent lanes, drivers and bicyclists will be

guided by lane markings up to the entrance

line. At the yield point, they will continue

along their natural trajectory into the

circulatory roadway. The speed and

orientation of the design vehicle at the entrance

line determines what can be described as its

natural path. The geometry of the exits also

affects the natural path that the design vehicle

travels. The natural path of two vehicles are

not to overlap, see NCHRP Guide 2,

Section 6.7.2.

(13) Splitter Islands.

Splitter islands (also called separator islands,

divisional islands, or median islands) will be

provided on all roundabouts. The purpose is to

provide refuge for pedestrians, assist in

controlling speeds, guide traffic into the

roundabout, physically separate entering and

exiting traffic streams, and deter wrongway

movements.

The total length of the raised island should be

at least 50 feet although 100 feet is desirable.

On higher speed roadways, splitter island

lengths of 150 feet or more is beneficial.

Additionally, the splitter island should extend

beyond the end of the exit curve to prevent


December 14, 2018

Figure 405.10 Roundabout Geometric Elements

NOTE:

This figure is provided to only show nomenclature and is not to be used for design details.


exiting traffic from crossing into the path of

approaching traffic. The splitter island width

should be a minimum of 6 feet at the pedestrian

crossing to adequately provide refuge for

pedestrians.

Posted speeds on the approach roadway greater

than or equal to 45 miles per hour require the

splitter island length, as measured from the

inscribed circle diameter, to be 200 feet. In

some instances, a longer splitter island may be

desirable. Concrete curb is to be provided on

the right side of the approach roadway equal to

the length of the splitter island from the

inscribed circle diameter.

(14) Access Control.

The access control standards in Index 504.3(3)

and 504.8 apply to roundabouts at interchange

ramp intersections. The dimensions shown in

Index 504.8 are to be measured from the

inscribed circle diameter.

Driveways should not be placed within

100 feet from the inscribed circle diameter.

(15) Lighting.

Lighting is required at all roundabouts. See

NCHRP Report 672 Chapter 8, the Traffic

Manual Chapter 9 as well as consult with the

District Traffic Safety Engineer.

(16) Landscaping.

Landscaping should be designed such that

drivers and bicyclists can observe the signing

and shape of the roundabout as they approach,

allowing adequate visibility for making

decisions within the roundabout. The

landscaping of the central island can enhance

the intersection by making it a focal point, by

promoting lower speeds and by breaking the

headlight glare of oncoming vehicles or

bicycles. It is desirable to create a domed or

mounded central island, between 3.5 to 6 feet

high, to increase the visibility of the

intersection on the approach. Contact the

District Landscape Architecture Unit to

provide technical assistance in designing the

roundabout landscaping. See Chapter 900 for

additional Landscape Architecture

requirements.

(17) Vertical Clearance.

The vertical clearance guidance provided in

Index 309.2 applies to roundabouts.

(18) Drainage Design.

See Chapter 800 to 890 for further guidance.

(19) Maintenance.

Contact the District Maintenance Engineer and

appropriate Regional Manager for maintenance

strategies and practices including seasonal

operations, maintenance resources, and

specialized equipment. Maintenance

responsibilities may also include multiple state,

county, and city agencies where coordination

of maintenance efforts and funding is needed.

Consider maintenance of the central island.

Provide a maintenance vehicle pullout within

the central island beyond the truck apron, so

maintenance vehicles will not conflict with

circulating trucks.

(20) Snow Areas.

In climate regions where snowfall requires the

use of snow removal equipment, consider the

equipment to be used. Design ICD’s as well as

entrance and exit geometry to accommodate

snow removal equipment and plow limitations.

Check with District Maintenance for their

requirements and limitations. Geometric

elements to consider that facilitate snow

removal are; mountable curb, tapering the ends

of curbs down to allow plows to ride over

curbs, plowing accommodation in both

directions, providing snow storage space

within the central island, and providing

minimum entry/exit widths to accommodate

the plow blade. Mountable curb may be used

if sidewalk/shared use path is not contiguous to

the curb. Provide a planter or textured

pavement between the path and the roadway.

Snow storage areas must be designed to

prevent snow melt from entering the circulating

lanes where it can freeze. Snow storage areas

must not block pedestrian paths.


December 14, 2018

(21) Utilities.

Utility access openings (manholes) should not

be located within the traveled way within the

boundary of the roundabout. Roundabouts do

not have shoulders to accommodate traffic

while manholes are accessed. Manholes

should not be allowed within the circulating

roadway to avoid closing down the intersection

during access. If a manhole is absolutely

necessary within the boundary of the inscribed

diameter, place it in the central island and off

of the truck apron. Provide a maintenance

vehicle pullout to allow access to the manhole

without blocking truck traffic.

Topic 406 - Ramp Intersection Capacity Analysis

The following procedure for ramp intersection

analysis may be used to estimate the capacity of any

signalized intersection where the phasing is

relatively simple. It is useful in analyzing the need

for additional turning and through traffic lanes. For

a more complete analysis refer to the Highway

Capacity Manual.

(a) Ramp Intersection Analysis--For the typical

local street interchange there is usually a critical

intersection of a ramp and the crossroads that

establishes the capacity of the interchange. The

capacity of a point where lanes of traffic

intersect is 1500 vehicles per hour. This is

expressed as intersecting lane vehicles per hour

(ILV/hr). Table 406 gives values of ILV/hr for

various traffic flow conditions.

If a single-lane approach at a normal intersection

has a demand volume of 1000 vph, for example,

then the intersecting single-lane approach

volume cannot exceed 500 vph without delay.

The three examples that follow illustrate the

simplicity of analyzing ramp intersections using

this 1500 ILV/hr concept.

(b) Diamond Interchange--The critical intersection

of a diamond type interchange must

accommodate demands of three conflicting

travel paths. As traffic volumes approach

capacity, signalization will be needed. For the

spread diamond (Figure 406A), basic capacity

analysis is made on the assumption that

3-phase signalization is employed. For the tight

diamond (Figure 406B), it is assumed that 4-

phase signal timing is used.

(c) 2 Quadrant Cloverleaf--Because this inter-

change design (Figure 406C) permits 2-phase

signalization, it will have higher capacities on

the approach roadways. The critical intersection

is shared two ways instead of three ways as in

the diamond case.

Table 406

Vehicle Traffic Flow Conditions at Intersections at Various Levels of

Operation

ILV/hr Description

< 1200:

Stable flow with slight, but acceptable delay.

Occasional signal loading may develop. Free

midblock operations.

1200-1500:

Unstable flow with considerable delays possible.

Some vehicles occasionally wait two or more cycles

to pass through the intersection. Continuous backup

occurs on some approaches.

1500 (Capacity):

Stop-and-go operation with severe delay and heavy

congestion(1). Traffic volume is limited by

maximum discharge rates of each phase.

Continuous backup in varying degrees occurs on all

approaches. Where downstream capacity is

restrictive, mainline congestion can impede orderly

discharge through the intersection.

NOTE:

(1) The amount of congestion depends on how much the

ILV/hr value exceeds 1500. Observed flow rates will

normally not exceed 1500 ILV/hr, and the excess

will be delayed in a queue.


Figure 406A Spread Diamond


December 14, 2018

Figure 406B Tight Diamond


Figure 406C Two-quadrant Cloverleaf

HIGHWAY DESIGN MANUAL 400-1 March 7, 2014...402.1 Capacity Adequate capacity to handle peak period...

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